The present invention relates to improved core-shell polymers of the type disclosed in concurrently filed U.S. Patent Application Ser. No. 115,824 filed Jan. 28, 1980 entitled "CORE-SHELL POLYMERS AND PROCESS"; and more particularly, the invention relates to core-shell polymers of this type which have superior properties as screen print binders.
In the above-identified, related application, there is disclosed a new organic high polymer composition and a process for preparing it. The composition disclosed therein comprises discrete particles having cores of one polymer completely covered with shells of a different polymer. The shell will preferably be a methyl methacrylate, ethyl methacrylate, butyl methacrylate, or mixed alkyl acrylate and/or alkyl methacrylate polymer. The core will proferably be a styrene-butadiene (SBR) copolymer. By properly selecting the shell monomer, core particle size and reaction conditions to provide a core/shell number of greater than 10, it is possible to concentrate the shell polymer on the outside of the core and leave the center of the core free from shell polymer. These polymers were suggested for use as screen print binders and those having styrene-butadiene copolymer cores and polyalkyl acrylate/methacrylate shells were specifically found usable for this purpose because they exhibit better cyclic aging and redispersion properties than the styrene-butadiene copolymer by itself and are less expensive than the polyalkyl acrylate/methacrylate alone.
Screen printing is a process for applying patterns to textile materials. According to this procedure, a rotary drum having a screen printing mask similar to a silk screening mask, is brought into contact with a fabric and has a composition comprising an aqueous dispersion of pigment and binder applied to the outside of the mask. The composition is applied in a predetermined pattern by virtue of the printing mask and the patterned fabric is then heated to fix the design on the fabric.
While screen print binder compositions comprising core-shell polymers of the type described in the above-identified application have a desirable balance of cyclic aging, redispersibility and cost, experience has shown that most of the compositions disclosed therein cause the textile material to which they are applied to become unduly stiff. It is desirable in many situations, and essential in some, that the printed fabric be pliable and have good draping characteristics.